1. Field of the Invention
The present invention relates to integrated circuit packages, and more particularly, to a die attach material for a Tape Ball Grid Array (TBGA) or other flexible circuitry package.
2. Description of the Related Art
In the last few decades, the electronics industry has literally transformed the world. Electronic products are used by, or affect the daily lives of, a large segment of the world's population. For example, telephones, television, radios, Personal Computers (PCs), laptop PCs, palmtop PCs, PCs with built-in portable phones, cellular phones, wireless phones, pagers, modems and video camcorders, are just a few of the electronic products that have been developed in recent years and which have been made smaller and more compact, while providing more and/or enhanced functions than ever before. The integrated circuit (IC) chip, and the more efficient packaging of the IC chip, has played a key role in the success of these products.
The IC chip is not an isolated island. It must communicate with other chips in a circuit through an Input/Output (I/O) system of interconnects. Moreover, the IC chip and its embedded circuitry are delicate, and must therefore be protected in a package that can both carry and protect it. As a result, the major functions of the IC package are: (1) to provide a path for the electrical current that powers the circuits on the chip; (2) to distribute the signals on to and off of the chip; (3) to remove the heat generated by the circuit; and (4) to support and protect the chip from hostile environments.
As ICs become more complex, and printed circuit boards become more crowded, IC packages continually need more leads or pins while their footprints consume smaller and smaller areas. In an effort to meet these demands, developers created the ball grid array (BGA) package.
A typical BGA package includes an IC affixed to a flexible polyimide tape. A very thin conductor or wire bond connects a pad on the IC to a conductive trace on the polyimide tape. The conductive trace is routed to a solder ball. The solder ball is one of an array of solder balls that connect to the opposite side of the polyimide tape and protrude from the bottom of the BGA package. These solder balls interconnect with an array of pads located on a substrate, such as a printed circuit board. Accordingly, the typical BGA package electrically connects each pad on an IC to a pad on a printed circuit board.
A variation of the BGA package that has been introduced recently is the Area Tape Automated Bonding (ATAB) Ball Grid Array (BGA) package, or more commonly referred to as simply the Tape Ball Grid Array (TBGA) package. The TBGA package advantageously provides high lead counts, as well as a thin, lightweight, high electrical and thermal performance, and a BGA surface mount. The conventional TBGA package consists of a tape sandwiched between a polyimide dielectric. At least one layer of the tape is formed into traces or conductors that interconnect a chip to a printed circuit board (PCB). See John H. Lau (Ed.), Ball Grid Array Technology, Chapter 14, “Area Tape Automated Bonding Ball Grid Array Technology” (McGraw-Hill, 1995), incorporated herein by reference.
Provided between the chip and solder ball array in many IC packages such as the TBGA package described above is an elastomer compliant layer. Such a layer is found, for instance, in Tessera's Micro Ball Grid Array (μBGA) package. See John H. Lau (Ed.), Ball Grid Array Technology, Chapter 1, “A Brief Introduction to Ball Grid Array Technologies” (McGraw-Hill, 1995), incorporated herein by reference. The elastomer layer is provided between the die and the solder balls to absorb pressure between the two, especially during mounting. The compliant die attach interface thus applies less stress to the solder balls to resist cracking. This compliant layer desirably has a super low modulus and large coefficient of thermal expansion (CTE). For instance, a compliant elastomer layer having a modulus of less than 10 ksi and a coefficient of thermal expansion (CTE) of greater than 200 ppm/° C. has been used. Other compliant layers having a modulus in the range of 0.5 and 10 ksi and a CTE in the range of 200 to 400 ppm/° C. have also been used.
One drawback of this design is that it induces a fatal failure mode. Because of the elastomer layer's high CTE and compliance, as compared to the die, during thermal exposure the elastomer expands and shrinks more rapidly than the die. This creates stress on the conductive leads connecting the solder ball array to the die, especially at the pads connecting the die to the leads, and causes breakage of the leads from the die. This failure mode is known as the heel break. The loss of electrical connection between the IC pads and the conductive traces results in lower yield rates and increases the overall cost of package manufacture.